1. Field of the Invention
The invention relates generally to cooling of integrated circuits. More particularly, the invention relates to a thermally coupling electrically decoupling cooling-device to cool a self-heating electrically conductive line by transferring heat from the line to a semiconductor substrate while blocking flow of current from the line to the substrate, and to an integrated circuit containing the cooling device.
2. Background Information
Many integrated circuits include multi-layer electrical interconnect structures to provide power and electrical signals to logic elements such as transistors located on a semiconductor substrate. As will be explained further below, these interconnect structures may become heated during operation, due to typically small electrical resistances, and the heat so generated can lead to electromigration that degrades the performance and reliability of the integrated circuits.
FIG. 1 illustrates a cross section of an integrated circuit 100 having a two-metal layer electrical interconnect structure that contains a self-heating signal line 130 that, as a result of operation, converts a portion of an electrical current transmitted across the line into heat. The integrated circuit contains a dielectric material 120, a tortuous electrical interconnect structure 110 embedded in the dielectric material, and a semiconductor substrate 180 containing a transistor 170. The interconnect structure contains the signal line 130, via 140, a metal interconnect line 150, and a contact 160. The signal line resides within a single layer and receives electrical current representing a signal for the transistor. The via is in electrical contact with the signal line to receive the signal and provide it to the metal interconnect line located in a sublayer. Similarly, the contact is in electrical contact with the metal interconnect line to receive the signal and provide it to the transistor.
The signal line may generate heat as a result of the electrical current and signals transmitted over it. Typically this is a result of small but non-zero electrical resistances within the signal line that convert a portion of the electrical current energy into heat. The amount of heat generated by the signal line may be related to the square of the current carried by the line multiplied by the electrical resistance of the signal line material. This form of resistive self-heating is well known. Since the signal line is surrounded by dielectric material, which typically has a low thermal conductivity, the heat is unable to dissipate and the temperature of the signal line increases. Such an increase in temperature may promote electromigration that may degrade the performance and reliability of the interconnect lines and the integrated circuit.
Electromigration is the unintended movement of metal atoms of the signal line as a result of frictional forces imposed by electrical current and may lead to poor reliability and integrated circuit failure. Such movement may cause troughs to form at a start of the signal line where material is removed and hills to form at an end of an interconnect line where the moved material accumulates. This can weaken the line, rupture the line, and cause the integrated circuit to fail. Additionally, metal atoms may diffuse into the dielectric material creating unintended non-via electrically conductive pathways between layers that can electrically short. Accordingly, increases in signal line temperature may cause increased electromigration and decreased integrated circuit performance and reliability.
One way to reduce electromigration is to reduce current and current density in the signal line so that self-heating decreases. However, this poses significant limitations on integrated circuit performance. For example, this may cause the current levels to decrease to a point where the transistor switches slowly and performance of the integrated circuits compromised.
Recently, the significance of electromigration has increased due to the technological scale down and shrinkage of integrated circuits and interconnect structures. Shrinking a signal line reduces the cross sectional area, which causes higher current densities and greater electrical resistance per unit length. This higher electrical resistance may cause more heat to be generated. Additionally, reducing the cross sectional area decreases the amount of heat transfer area in contact with the surrounding dielectric material that is available for heat dissipation. This problem becomes even worse when low dielectric constant materials having a dielectric constant lower then silicon dioxide are used, since these materials often have even lower thermal conductivities that prevent heat dissipation.